Can you prefix the #'(complicated ..) expression with a form that forces all lazy computations so that complicated itself remains the same?
(let () (force-all) #'(complicated (form e.v))) where force-all is defined via a begin in the slow syntax class? -- Matthias On May 31, 2013, at 2:47 AM, Eric Dobson wrote: > I'm working on code (TR optimizer) that needs to match some > expressions and then compute an attribute based on the expression. I > would like to abstract this out as a syntax class. Example is below: > > #lang racket > (require syntax/parse) > > (define-syntax-class slow > (pattern e:expr > #:with v (begin (displayln 'slow) #''e))) > > (syntax-parse #'(x 2) > ((e:slow 1) #'(complicated (form e.v))) > ((e:slow 2) #'(complicated (form e.v)))) > > The issue is that computing the attribute is slow/expensive/effectful > and so I only want to do the calculation in the case that I use the > result. One solution is to attach a thunk as the attribute value, but > this requires changing #'(complicated (form e.v)) to #`(complicated > (form #,((attribute e.v)))) which quickly becomes less readable with > many such attributes in the final syntax object. Is there a way to do > this in a lazy manner that does not complicate the use of the syntax > class? I'm fine adding complexity to the creation of the syntax class > because it is a one time cost versus the many places where the syntax > class would be used. > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
